1. Field of the Invention
The present invention relates to bases for photographic sensitive materials comprising polyolefin coated paper.
2. Description of the Prior Art
Since polyolefin synthetic resins such as polyethylene, polypropylene, etc., have an excellent waterproofing property and an excellent moisture-proofing property, papers have been coated with these polyolefins in many fields in order to waterproof them. In photographic sensitive materials so-called waterproof printing paper prepared by applying photographic emulsion layers to a paper base the surfaces of which are coated with polyolefin has been used to shorten the development time. By the use of the above-described waterproof printing paper, the time required for development processing is 1/2 to 1/3 that required using baryta paper as the base, because the time required for carrying out washing and drying after development processing can be remarkably reduced.
An extrusion coating process has generally been adopted in producing bases for waterproof printing paper as a process for applying polyolefin to paper. Namely, melted polyolefin is flown as a film onto a surface of the paper while moving the paper, and the polyolefin and the paper are then allowed to adhere to one another by adaptation. However, since the polyolefin is a non-polar high molecular material, its adhesion to paper is not always good, and coating papers in which the adhesion of the polyolefin to the paper is insufficient are sometimes obtained depending upon the raw materials used or the manufacturing method. It has been known that, in the case of using such coating paper as a base for printing paper, the following problems occur in practice.
Namely, when a paper in which the adhesion of the polyolefin to the paper is insufficient is used as the base for a waterproof printing paper, the polyolefin separates from the paper at the edges (cut ends) of the printing paper due to friction, etc., and ruins the appearance of the printing paper. Also, particularly, when separation at the edges occurs in a wet state during a development processing, the processing solution is absorbed by the paper and, consequently, rapid processing often cannot be carried out.
Further, when the waterproof printing paper after conclusion of the development processing is cut to a desired size by a special knife, if the adhesion of the polyolefin to the paper is insufficient, whisker-like stretched parts of polyolefin are often formed at the cut ends causing a poor appearance of the cut ends. In some cases, the stretched polyolefin is not cut completely and two or more of cut printing papers link in a row. Therefore, when using the polyolefin coated paper as a base for printing paper, sufficient adhesion of the polyolefin to the paper is a very important requirement.
The simplest process for improving adhesion of polyolefin to paper comprises carrying out extrusion coating at a resin temperature of 320.degree. C. or more which is just below the decomposition temperature of the polyolefin. However, a part of the polyolefin decomposes even at this temperature to result in not only deterioration of operation efficiency but also deterioration of tone, strength, and other properties of the resulting coated paper and, the thermal decomposition products have a bad influence upon the photographic emulsions which is fatal for photographic paper bases.
Generally, since photographic paper has a smooth surface, which is different from kraft paper, a remarkable improvement in adhesive strength cannot be obtained even if the above-described process is utilized or activation processing of the surface of paper such as by corona discharge, etc., is carried out. As an effective process for improving the adhesion of a polyolefin to paper, a material which exhibits good adhesive property for both the paper and the polyolefin has been applied to a surface of the paper before application of polyolefin (a so-called primer treatment) by extrusion coating. However, polyethyleneimine and low molecular polyethylene emulsions which are known effective primers all have a bad influence on the photographic emulsions, and, therefore, they are unsuitable as raw materials for photography. Thus it is difficult to use the primer treatment in producing bases for waterproof printing paper.
As primers for extrusion coating photographic paper with a polyolefin, there are copolymers comprising ethylene and salts of acrylic acid (or methacrylic acid) as constituents as described in Japanese Patent Application (OPI) No. 15423/74 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application"). However, in the case of using these primers, sufficient adhesive strength can not be obtained if the extrusion temperature of the polyolefin is reduced or the thickness of the polyolefin is reduced, and the requirements of improving production aptitude at low extrusion temperatures or of reducing material cost, etc., cannot be satisfied. Accordingly, primers are needed which satisfy these requirements and produce high adhesion.
In consideration of the above-described facts, studies directed to primers which exhibit an excellent improvement in adhesion of the polyolefin to photographic paper and do not have a bad influence upon photographic emulsions. As a result, bases for waterproof printing paper having an excellent adhesive property for polyolefins have been found and constitute the present invention.